Machines may include one or more hydraulic power systems to drive one or more loads. The load may be a work implement on the machine or it may be a drive component that provides propulsion for the machine itself. For example, in a machine drive train, a hydraulic power system, also known as a hydrostatic transmission, may be used in lieu of a mechanical transmission.
A hydraulic power system may include a variable displacement hydraulic pump and a hydraulic motor, which may also have variable displacement, that are connected together in a closed loop configuration. Fluid pumping through the hydraulic motor can cause it to spin an output shaft to thereby move a load such as a drive mechanism, such as a wheel or track, or a work implement. By varying the displacement of the pump, the amount of fluid pumped to the hydraulic motor may be controlled. This can be in response to a received operator input. For example, when an operator depresses an accelerator pedal to indicate a desire for more speed or torque of a drive mechanism, a discharge of the pump (flow and/or pressure) is proportionally increased.
To protect components of the transmission from damage, operation of the pump and/or motor is commonly limited according to pressure. Pressure may build in a hydraulic power system that functions to power the drive mechanism of a machine when the machine encounters an external resistance such as when pushing on something that is heavy or substantially immovable, like a large pile of earth. When the machine meets the resistance of the large pile of earth, the forward travel of the machine may be slowed or stopped, which, in turn, slows or stops the hydraulic motor that drives the drive mechanism. This substantially inhibits the flow of fluid through the motor. However, the variable displacement pump may continue to pump fluid to the hydraulic motor resulting in a build-up of pressure in the system.
One way to relieve this kind of pressure build up is with a cross-over relief (COR) valve, which may permit hydraulic fluid to flow (i.e., cross over) from the high pressure side of the circuit over to the low pressure side. While a COR valve can prevent spikes in pressure, continued flow across a COR valve can cause significant heating of the hydraulic fluid due to the pressure drop of the fluid as it passes from the high pressure side to the low pressure side of the system. Moreover, flow across a COR can be an inefficient use of energy since the flow through the valve is not productive, that is, not being used in a productive manner, such as, for example to turn the hydraulic motor.
Another common way to provide pressure relief is with an electronic pressure override (EPOR) system. An EPOR system senses system pressure and acts to reduce the displacement of the variable displacement pump, and thus reduce the amount of fluid being pumped to the hydraulic motor (or implement actuator), when the pressure exceeds a certain amount. Many hydraulic systems include EPOR systems in addition to COR valves.
An example of an EPOR system with COR is disclosed in U.S. Pat. No. 6,202,411 (the '411 patent). The '411 patent discloses a system that adjusts the discharge flow rate of a hydraulic pump when the system is held at a predetermined pressure for a predetermined period of time and when a specific operational condition of the system is sensed. Some of the disclosed operational conditions include use of a specific type of work implement, a high revolution condition of the engine and an operator selected work mode.
While effective to reduce pressure in the system, EPOR systems that are based on pressure control can be difficult to tune in a way that yields a consistent, intuitive feel to an operator of the machine. In particular, since the pressure control can be quite sensitive, when the EPOR system activates to reduce the pressure in a system, the pressure can momentarily overshoot and then drop dramatically causing an operator to feel as though the machine has suddenly stopped pushing. Moreover, an EPOR system based on pressure control can be relatively stiff to control and must be set to a pressure that is less than the upper limit of the system meaning that the machine may produce less than its maximum capable performance.